High blood pressure (BP), or hypertension (HTN), is a major, heritable cardiovascular risk factor. Several population studies have shown that red blood cell (RBC) traits increase the risk of HTN, coronary heart disease (CHD) and all-cause mortality. Many specific biologic interactions between erythrocytes and the vasculature have been defined, such as regulation of endothelial nitric oxide-mediated vasodilation, but the mechanisms underlying the epidemiologic relationship between RBC traits, HTN and CHD are poorly defined. Concurrently, despite intense investigation into genetic associations with BP in the population, we have yet to define the mechanisms underlying the majority of currently known genetic associations. Genome-wide association studies (GWAS) have identified loci that are common for RBC and BP traits, which is consistent with pleiotropy, but additional investigation is needed to explore whether these genes are directly influencing these traits concordantly, or whether other mechanisms, such as mediation or multiple genes in linkage, are responsible. In this proposal, we will test the overall hypothesis that genetic determinants of RBC traits influence BP and that through statistical analyses of pleiotropy, we may more precisely define the contributions of RBC traits to BP, which are relevant for the outcomes of HTN and CHD. We will conduct statistical analyses in the Atherosclerosis Risk in Communities Study, the Rochester Family Heart Study and the Jackson Heart Study with phenotypes and genetic sequence data in population cohorts as well as functional experiments to evaluate target identified in the statistical analyses. Accordingly, the Aims of this proposal are to: (1) evaluate genetic pleiotropy for RBC and BP traits in population samples; (2) use Mendelian randomization approaches to evaluate the effect of the identified variants on HTN and CHD outcomes; and (3) use functional assays to model the effect of specific genes and variants in vitro and ex vivo. Through the logical progression of these Aims, we will dissect the mechanisms of pleiotropic genetic associations with RBC and BP traits, and we will evaluate their functional and clinical impact on major cardiovascular outcomes.